A mate of mine has done the same conversion and is running a GTI backbox. Think he got a standard cat back off ebay cheap and used that.
Just spoke to him and said it was just the backbox he fitted but it had to be welded on. Someone may know another solution though
Ive got a sleeve and clamps holding my A3 box on motor factors £10http://www.ebay.co.uk/itm/2-25-60MM-HEAVY-DUTY-EXHAUST-SLEEVE-CLAMP-COLLAR-PIPE-CONNECTOR-JOINER-ADAPTER-/350885712904 This style but a tad cheaper